It could be they have a clipping service that checks for copyright violations. Or maybe just someone at the paper doing searches. I have noticed on occasion when I do a Google search that I sometimes get a hit on Free Republic.
Posted articles sometimes contain the author's email address,
and I assume they must sometimes receive emails from our forum participants.
While it hasn't happened often, I can recall instances where the author has registered and entered into the discussion of their article. (I vaguely remember this happening with one of the Pittsburgh papers, but can't recall whether it was the P-G or the Tribune-Review.)
Anyway, that is another avenue by which they become aware of our discussions.